Expanded foams, particularly polystyrene foams, are widely used in the packaging and construction industries. Unfortunately, flame-retardant polystyrene with acceptable physical properties is difficult to manufacture. Halogenated flame-retardant additives used in flame-retardant polymers can leach out of the foam during burning and liberate harmful hydrogen halide vapors. In addition, many halogenated flame-retardant additives are unacceptable because they interfere with the polymerization, resulting in products that contain an undesirably high level of residual monomer. Chemical incorporation of flame retardants into the polymer backbone can give polymer products with good flame resistance and a reduced tendency to liberate harmful vapors. Unfortunately, ethylenically unsaturated monomers that incorporate flame-retardant moieties, particularly inexpensive, nonhalogenated ones, are in short supply.
Copolymers of vinyl phosphonic acid derivatives with olefinic compounds are known, and many variations are described in the patent literature. Thermoplastic, expandable beads of these copolymers useful for making foamed, flame-resistant articles have not been previously described.
U.S. Pat. No. 2,439,214 teaches copolymers of .alpha.,.beta.-unsaturated phosphonic acids and diesters with mono-ethylenic compounds. The reference shows (Example IV) the bulk copolymerization of dimethyl 1-propene-2-phosphonate with styrene to produce a clear, colorless resin.
U.S. Pat. No. 2,743,261 teaches copolymers of .alpha.- and .beta.-phosphonate styrenes (diesters) and monoethylenically unsaturated compounds. The copolymers are described as clear, tough, flame-resistant, hard resinous copolymers that can be molded into shaped objects or spun into fibers. The reference shows (Example I) bulk copolymerization of styrene and diethyl 1-phenyl vinyl phosphonate.
U.S. Pat. No. 3,726,839 teaches crosslinked polymers of a bis(hydrocarbyl) vinylphosphonate, a polyfunctional ethylenically unsaturated monomer such as divinylbenzene, and optionally a monofunctional vinyl comonomer. The compositions are useful as flame-retardant polymers or as polymer additives.
U.S. Pat. No. 3,763,122 teaches copolymers of styrene or acrylamide and phenyl vinyl phosphonic acids useful for improving the burst strength of paper.
U.S. Pat. No. 3,991,134 teaches copolymers of bis(hydrocarbyl) vinylphosphonates with halogen-containing .alpha.,.beta.-ethylenically unsaturated (vinyl) monomers. The reference teaches that mono(alkyl) acid vinylphosphonates can also be used.
U.S. Pat. No. 3,993,715 teaches a process for making fire-retardant polymers of bis(hydrocarbyl) vinyl phosphonates. A monoethylenically unsaturated monomer is added as a chaser to complete reaction of the phosphonate monomer.
U.S. Pat. No. 4,035,571 teaches copolymers of a bis(hydrocarbyl) vinylphosphonate, a monomer having one ethylenically unsaturated bond, and acrylic or methacrylic acid. The compositions are useful in coatings or as flame-retardant additives for thermoplastics. The reference teaches that these copolymers may be prepared by aqueous suspension polymerization. As shown in Example 2 of the reference, unreacted phosphonate monomer is recovered from the polymerization reaction mixture if the acrylic monomer is omitted.
U.S. Pat. No. 4,444,969 teaches copolymers of a vinyl-substituted aryl hydrocarbon monomer, an imide derivative of a cyclic anhydride, and a bis(hydrocarbyl) vinylphosphonate. The solid compositions are useful as fire-retardant additives.
A number of papers in the Russian literature describe the preparation of copolymers of .alpha.-phenylvinylphosphonic acid (PvPA) with vinyl monomers such as methyl methacrylate and styrene. Spherical granules useful as ion exchangers can reportedly be prepared by suspension polymerization if a crosslinking agent such as divinylbenzene is included (See, for example, Plast. Massy, No. 8 (1966) 24, Plast. Massy, No. 2 (1966) 17, Chem. Abstr. 72 32299g, Chem. Abstr. 83 132379d, and Chem. Abstr. 83 60019m). Such crosslinked copolymer beads are not expandable and therefore not suitable for use in the preparation of foamed articles. The radical copolymerization of dialkylvinyl phosphonates with styrene and methyl methacrylate was studied by Levin et al. (Polym. Sci. USSR 24 (1982) 667).
A recent paper (Macromolecules 22 (1989) 4390) describes a copolymer including a vinylphosphonic acid monoalkyl ester. Monoalkyl esters of this type have been prepared by basic hydrolysis of the corresponding diesters in dioxane (J. Organometal. Chem. 12 (1968) 459).
None of the references teaches a process wherein vinyl aromatic monomers and vinyl phosphonic acids are copolymerized in the absence of a crosslinking agent to give thermoplastic polymer beads. In addition, none of the above references teaches a process for the preparation of foamed articles from thermoplastic expandable copolymer beads made by suspension copolymerization of a vinyl aromatic monomer and a vinyl phosphonic acid derivative.